Idle State Signalling Reduction (ISR) is a technology to reduce idle state signalling in an Evolved Packet System (EPS) so as to alleviate a signalling load between a User Equipment (UE) in an idle state and a network.
A Serving GPRS Support Node (SGSN, where GPRS stands for General Packet Radio Service) is a network entity in a Universal Terrestrial Radio Access Network (UTRAN), and a Mobility Management Entity (MME) is a network entity in an Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
When the UE registers with both the MME in the E-UTRAN and the SGSN in the UTRAN and activates ISR, the UE in an idle state will not perform signalling interaction with the network side while the UE resides and moves in a Tracking Area (TA) in a list of Tracking Area IDs (TAIs) registered in the E-UTRAN and a Routing Area (RA) registered in the UTRAN.
Activation of ISR is controlled at the network side, and when the UE in an idle state initiates a Tracking Area Update (TAU) or Routing Area Update (RAU) procedure in the E-UTRAN or the UTRAN, the MME or the SGSN instructs the UE to activate or deactivate ISR in a TAU Accept message or an RAU Accept message, and the UE activates or deactivates ISR in response to the instruction. An initial value of a Temporary Identity used in Next update (TIN) is set by the UE upon successful attachment. The UE resets the TIN to a Globally Unique Temporary Identity (GUTI) regardless of original value thereof upon successful initial attachment to the E-UTRAN. The UE resets the TIN to a Packet Temporary Mobile Station Identity (P-TMSI) regardless of original value thereof upon successful initial attachment to the GERAN/UTRAN. The UE has to set a locally stored TIN to a GUTI or a P-TMSI regardless of original value thereof upon reception of the TAU Accept message or the RAU Accept message instructing ISR to be deactivated. With the TAU Accept message instructing ISR to be activated, the UE decides setting of the TIN dependent upon the value of the TIN. If the value of the TIN is a P-TMSI or a Radio Access Type related TMSI (RAT-related TMSI), the TIN is set to the RAT-related TMSI, and if the value of the TIN is a GUTI, the value of the TIN is set to the GUTI. With the RAU Accept message instructing ISR to be activated, the UE decides setting of the TIN dependent upon the value of the TIN if the value of the TIN is a GUTI or an RAT-related TMSI, the TIN is set to the RAT-related TMSI, and if the value of the TIN is a P-TMSI, the value of the TIN is set to the P-TMSI. Noted the value of the TIN is merely for the purpose of distinguishing.
In this context, “area update” broadly refers to “tracking area update” or “routing area update”. The following description will be given by way of an example in which a UE firstly is attached to a network over a UTRAN and then accesses an E-UTRAN to initiate a TAU procedure. As illustrated in FIG. 1, the TAU procedure is as follows.
The UE determines that a TAU trigger condition is satisfied when the UE accesses a TA in the E-UTRAN from an RA in the UTRAN and the triggers a TAU to be started (the step S101); the UE initiates a Tracking Area Update Request to an MME (the step S102 to the step S103); since the UE is registered with an old S4 SGSN (one type of SGSN supporting ISR; and SGSN in this context generally refers to the S4-SGSN), the new MME retrieves context information of the UE from the old SGSN (the step S104, the step S105 and the step 107); then the MME instructs a Serving Gateway (GW)/a Packet Data Network Gateway (PDN GW) to modify bearer information of the UE (the step S109 to the step S113); the MME transmits a Location Update Request to an HSS to instruct the HSS to cancel location information of the UE in the old SGSN (the step S114 to the step S116); the old SGSN releases an Iu interface connection (the step S117 and the step S118); the HSS returns a Location Update Response message to the MME (the step S119); and the MME returns a Tracking Area Update Accept Message to the UE to thereby finish the tracking area update (the step S120 to the step S121). The MME may re-authenticate the UE in the TAU procedure (the step S106).
ISR-related aspects in the foregoing TAU procedure are stipulated in the existing specification (TS23.060 and TS23.401) as follows:
In the step S105, the SGSN transmits a Context Response message to the MME to instruct ISR to be activated; and
In the step S107, the SGSN receives a Context Acknowledgement message, transmitted from the MME, instructing ISR to be activated, and then the SGSN has to maintain a context of the UE.
The foregoing stipulations may result in a problem which will be described below in connection with several related timers.
A mobile reachable timer of the UE is maintained at the network side, the mobile reachable timer is started when the UE enters an idle state, and the mobile reachable timer is stopped at the network side when the UE has a Non-Access Stratum (NAS) signalling connection established with the network side (the UE enters a connected state); and
An implicit detach tinier of the UE is maintained at the network side, and if the mobile reachable timer expires and the UE has no NAS signalling connection established with the network side, the implicit detach timer is started at the network side. An implicit detach procedure is initiated at the network side after the implicit detach timer expires. The implicit detach timer is stopped at the network side when the UE has an NAS signalling connection established with the network side.
The UE maintains a periodic TAU timer, or a periodic RAU timer, or a periodic TAU timer and a periodic RAU timer dependent upon an access scheme and an ISR state.
The UE with a UTRAN access has to maintain a periodic RAU timer when ISR is deactivated. The UE has to initiate a periodic RAU procedure when the periodic RAU timer expires. The UE with an E-UTRAN access has to maintain a periodic TAU timer when ISR is deactivated. The UE has to initiate a periodic TAU procedure when the periodic TAU timer expires. The periodic TAU timer, or the periodic RAU timer, or the periodic TAU timer and the periodic RAU timer are started when the UE enters an idle state and stopped when the UE has an NAS signalling connection established with the network.
The UE has to maintain both of the timers when ISR is activated. If the periodic TAU timer expires when the UE resides in the UTRAN, the UE has to start an E-UTRAN Deactivate ISR Timer, and the UE has to initiate a TAU procedure immediately upon next entry into the E-UTRAN. When the E-UTRAN Deactivate ISR Timer expires, the UE has to deactivate ISR locally, that is, sets a TIN to a “P-TMSI”. The UE stops the E-UTRAN Deactivate ISR Timer upon entering an EPS Mobility Management-DEREGISTERED (EMM-DEREGISTERED) state or an EPS Mobility Management-CONNECTED (EMM-CONNECTED) mode. If the periodic RAU timer expires when the UE resides in the E-UTRAN, the UE has to start a GERAN/UTRAN Deactivate ISR Timer (GERAN stands for GSM EDGE Radio Access Network. GSM stands for Global System for Mobile communication, and EDGE stands for Enhanced Data rates for GSM Evolution), and the UE has to initiate an RAU procedure immediately upon next entry into the UTRAN. The UE has to deactivate ISR when the GERAN/UTRAN Deactivate ISR Timer expires. The UE stops the GERAN/UTRAN Deactivate ISR Timer upon entering a GPRS Mobility Management-DEREGISTERED (GMM-DEREGISTERED) state or a GMM-CONNECTED mode.
As stipulated in the existing specification, the mobile reachable timer has a slightly larger timer length (that is, a timing period as meant hereinafter; and a timer length, a timing period, and a timer value refer to the same meaning) than that of the periodic TAU timer or the periodic RAU timer. When ISR is activated, the Implicit Detach Timer at the MME has a larger timer value than that of the E-UTRAN Deactivate ISR Timer of the UE, and the Implicit Detach Timer at the SGSN has a larger timer value than that of the GERAN/UTRAN Deactivate ISR Timer of the UE.
The UE initiating the RAU has to include a different identity in an RAU request dependent upon setting of a TIN. The RAU request includes only a Packet Temporary Mobile Station Identity (P-TMSI) when ISR is activated. When ISR is deactivated, the RAU request includes only a P-TMSI if the TIN is set to the P-TMSI. If the TIN is set to a Globally Unique Temporary Identity (GUTI), the UE has to map the GUTI to a P-TMSI and include it in an RAU request message and shall also include the P-TMSI in the RAU request message if the UE is also provided with the P-TMSI, The SGSN can further search locally with the P-TMSI for a context of the UE upon reception of the request including the two temporary identities.
Thus the MME instructs the UE to activate ISR if the old SGSN still instructs the MME to activate ISR in the step S105 upon reception of a context request message from the MME (the step S104) after the Mobile Reachable Timer of the UE expires and before the Implicit Detach Timer expires. At this time, since the UE firstly is attached over the UTRAN and its TIN is set to a P-TMSI, the UE will set the TIN to an RAT-related TMSI when the TAU Accept message instructs ISR to be activated, if at this time the LIE goes from the E-UTRAN back to the UTRAN again and the UE determines that ISR is in an activated state, that is, the TIN is set to the RAT-related TMSI, the UE will initiate the RAU with the P-TMSI. At this time the Implicit Detach Timer maintained by the SGSN may have expired, and since the SGSN initiates the implicit detach procedure after the Implicit Detach Timer expires, the context of the UE is absent at the SGSN at that time, so the SGSN can not retrieve the context of the UE with the P-TMSI over the network. At this time the network will reject the RAU request of the UE, and the UE, has to reinitiate an attach procedure. The network can also reinitiate an authentication procedure. Inconsistency arises between the ISR state indication at the UE side and the ISR state at the network side (that is, a context maintenance condition of the UE at the network side), thus resulting in a certain delay in an access of the UE over the UTRAN.
Furthermore, the SGSN has to maintain the context of the UE upon reception of the Context Acknowledgement message from the MME as mentioned in the step S107. However, a specific period of time for which the context of the UE is maintained is absent in the specification. If the SGSN receives a Context Request message from the MME after the Mobile Reachable Timer of the UE expires and before the Implicit Detach Timer expires, instructs the MME to activate ISR and continues with maintaining the context of the UE after the Implicit Detach Timer expires, then a great deal of useless context information arises in the SGSN, and a mechanism to clean such useless information is absent.
The foregoing problem can be addressed in such a method that ISR is deactivated locally at the UE side. An underlying idea of this method is as follows:                1) In the UTRAN, the UE does not activate ISR or deactivates ISR locally if the periodic TAU timer of the UE expires when the network instructs the UE to activate ISR in the RAU. Thereafter the TAU request message will include two temporary identities of a P-TMSI and a GUTI when the UE accessing the E-UTRAN initiates the TAU        2) In the E-UTRAN, the UE does not activate ISR or deactivates ISR locally if the periodic RAU timer of the UE expires when the network instructs the UE to activate ISR in the TAU. Thereafter the RAU request message will include two temporary identities of a P-TMSI and a GUTI when the UE accessing the UTRAN initiates the RAU.        
The problem of inconsistency between the context maintenance condition indicated by the ISR state at the UE side and the real maintenance condition may still arise in the foregoing method, and either the TAU request or the RAU request will include two identities due to deactivation of ISR when the UE accesses from one network to another network, thus wasting an air interface resource to some extent; and the two temporary identities have to be received at the network side for processing, thus increasing signalling overhead at the network side and time delay in the TAU or RAU process.